1. Field of the Invention
The present invention relates to synchronization in general, and more specifically to adaptive FFT window synchronization in wireless OFDM systems.
2. Description of the Related Art
Advanced multimedia services continue to drive requirements for increasing data rates and higher performance in wireless systems. High performance communication systems such as those specified by the European terrestrial digital video broadcasting (DVB-T) standard and the Japanese integrated services digital broadcasting terrestrial standard (ISDB-T) may use communication methods based on Orthogonal Frequency Division Multiplexing (OFDM). The performance of an OFDM receiver may be sensitive to synchronization of a received sample sequence to a fast Fourier transform (FFT) window that converts the received sample sequence to a series of complex data values at a number of distinct sub-carrier frequencies. The DVB-T and ISDB-T standards may use a single-frequency network (SFN) technique, and performance of a wireless receiver in an SFN network may depend on the position of the FFT window relative to several simultaneously received signals from each of several transmitting antennas in the SFN network.
A received OFDM symbol in an OFDM system may consist of both data and pilot synchronization information transmitted on multiple sub-carriers multiplexed together and spanning multiple sample periods. Modulation and demodulation in an OFDM system may use an inverse fast Fourier transform (IFFT) at the transmitter and an FFT at the receiver respectively. At the transmitter a cyclic prefix of a section of the IFFT output for each OFDM symbol may be appended to the beginning of the OFDM symbol as a guard interval (GI) to provide some protection from inter-symbol interference (ISI). The length of the OFDM symbol before adding the guard interval may be known as the useful symbol period duration. At the receiver the cyclic prefix (guard interval) may be removed prior to the FFT demodulation by positioning appropriately an FFT window, whose size may be equal to the useful symbol period duration, along a received sample sequence. The FFT demodulation may transform the window of received time domain samples, in the received sample sequence, to a frequency domain (OFDM) symbol.
In a single frequency (or multi-path) network, the OFDM system may receive multiple, superimposed copies of a sequence of transmitted OFDM symbols. Each copy of the sequence of transmitted OFDM symbols may incur a different time delay and be scaled in amplitude by a different gain. The guard interval added to each OFDM symbol may provide flexibility to position the demodulation FFT window appropriately to minimize ISI and thereby improve performance. If a maximum delay spread between the beginning of the guard interval of the earliest received copy of a transmitted OFDM symbol and the beginning of the guard interval of the latest received copy of the transmitted OFDM symbol is less than the length of the guard interval, the FFT window may be positioned to eliminate ISI. For maximum delay spreads greater than the length of the guard interval, the FFT window may be positioned to minimize but not eliminate ISI. Prior art coarse synchronization methods may determine an FFT window position based on a correlation of the received data samples to estimate a location of the guard interval. Such coarse synchronization methods may not minimize ISI or maximize performance.
FIG. 1 illustrates an OFDM symbol sequence 100 including a series of OFDM symbols, each of useful symbol period duration Tu, appended by guard intervals of duration Tg. For example, guard interval 101 for OFDM symbol 102 repeats a last portion 103 of the OFDM symbol 102 before the beginning of the OFDM symbol 102. In a received sample sequence, the location of the guard intervals and useful symbol periods may be unknown at the receiver. An estimate of the location of the guard interval may be determined by correlating a first Tg length segment 104 of the received sample sequence with a second Tg length segment 105 separated by the useful symbol period duration Tu. As shown by plot 107, a correlation result may peak when the first Tg length segment 104 is positioned over the guard interval. The demodulator's FFT window may then be positioned over the useful symbol period (102 and 103), although this positioning may not maximize performance in an SFN or multi-path network as explained next.
FIG. 2 illustrates two received copies (200 and 201) of a transmitted OFDM symbol sequence, each copy incurring different time delays and different amplitude gains. The horizontal axis may indicate time, while the vertical height of each sequence may indicate relative amplitude gain. A correlation of a guard interval width segment of sequence 200 may result in the dotted line plot 203, while a correlation of a guard interval width segment of sequence 201 may result in the dashed line plot 204. As the sequence 200 may be received earlier, i.e. with less time delay, than the sequence 201, a peak in the correlation plot 203 also appears earlier than a peak in the correlation plot 204. A combined correlation plot 202 for both sequences received together, which the receiver may observe, may contain two different peaks 205 and 206. If the maximum delay spread exceeds the guard interval width, then positioning a receiver's FFT window based on either of these peaks may not minimize ISI or maximize receiver performance. (The delay spread as shown in FIG. 2 is less than the guard interval width, but if the OFDM symbol sequence 201 is delayed further in time, i.e. to the right, the peak 206 may be pushed outside of the guard interval of the OFDM symbol sequence 200.) Thus there exists a need for an improved system and method for adaptive synchronization to position an FFT window that may provide improved receiver performance in a multi-path or single frequency network.